I don´t know why there are German players in singles if the NOC didn´t nominate Barna and Weingärtner. Could they have been nominated by the ITF ?!

They probably got in through direct entry.

Quote:

Direct acceptance for the event was based on the men’s and women’s rankings of 14 June 2004 with the first 56 places in the men’s and women’s singles draw based solely on ranking, with a limit of four players per country. Two places in each singles draw were selected by a Tripartite Commission comprising the International Olympic Committee, National Olympic Committees and the ITF, while six places were selected by the ITF Olympic Committee based on a combination of ranking and geographic diversity.

Top 56 woman get in directly, if you take into account that there are people who can't go (like Raymond or Shaughnessy, since they are the 5th and 6th USA women), the list goes down to the top80s for direct entry.

Following the announcement of players accepted and qualified for the Tennis Event at the 2004 Olympic Games in Athens last week it is confirmed that Denisa Chladkova of the Czech Republic will not participate in the Women’s Singles Event.
Chladkova has been replaced by Mervana Jugic-Salkic of Bosnia and Herzegovina. The addition of Jugic-Salkic brings the number of participating countries to 54.